Abstract

This study describes the optimization of experimental conditions of two decomposition procedures for plant material namely, open-vessel wet digestion and ashing. Subsequent determination of cadmium, lead and chromium using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) was performed. The effectiveness of the optimized methods was tested by conducting recovery studies, precision tests and sensitivity tests. Finally, the two methods were compared on the basis of blank values, analysis time, sample weight, accuracy, precision and detection limit.
For open-vessel wet digestion method, nitric acid was found to be the most effective oxidant for decomposition while hydrogen peroxide was found to be the best combination reagent. Improvements in recoveries were achieved by optimizing the quantities of nitric acid and hydrogen peroxide; a 2:1 ratio was found to be optimum. However, increasing the amount of sample decreases the percent recovery of the three metals, due to organic matrix effect. The optimized method in general, is applicable to sample weight less than 2 g. Low temperature heating is required at the initial stage of decomposition, to avoid vigorous reaction leading to losses. For the remaining hours of digestion, a higher temperature could be used, provided that no spattering/bumping occurs.
For the ashing method, sulphuric acid was found to be a suitable chemical modifier for cadmium, with 550ºC as optimum ashing temperature and seven hours as optimum ashing time. For lead, sulphuric acid was also found to be a suitable modifier, with 600ºC as optimum ashing temperature and four to six hours as optimum ashing time. The optimum volume of sulphuric acid, required for 1 g of the plant material studied, water hyacinth (Eichhornia crassipes), was 2.0 to 2.5 mL. No modifier was found necessary for the complete recovery of chromium, for ashing at 600ºC for seven to eight hours. However, the effect of sample matrix on recovery is evident at high sample weights. The optimized method is applicable to sample weights less than 5 g. Charring is the most critical step of ashing; slow-continual or step-wise increase of temperature, during charring, is very important.
Lower blank values were achieved for ashing than for wet digestion. Ashing is more accurate than wet digestion. Large sample weights can be used for ashing. However, wet digestion has the advantage of short analysis time. Precision and detection limit are comparable for both methods.